Volatilization of iodine from nitric acid using peroxide

ABSTRACT

A method for removing radioactive iodine from nitric acid solution by adding hydrogen peroxide to the solution while concurrently holding the solution at the boiling point and distilling hydrogen iodide from the solution.

United States Patent 1191 Cathers et al.

[4 1 Oct. 21, 1975 VOLATILIZATION OF IODINE FROM NITRIC ACID USING PEROXIDE [75] Inventors: George I. Cathers; Calvin J.

Shipman, both of Knoxville, Tenn.

[73] Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC.

[22] Filed: Sept. 11, 1973 [21] Appl. No.: 396,336

[52] U.S. Cl 423/249; 423/500; 252/301.1 R [51] Int. Cl. COIF 13/00 [58] Field of Search 423/249, 500; 252/30l.1 R

Primary Examiner- Stephen J. Lechert, Jr. Attorney, Agent, or FirmJohn A. I-loran; David S. Zachry; John B. Hardaway [57] ABSTRACT A method for removing radioactive iodine from nitric acid solution by adding hydrogen peroxide to the solution while concurrently holding the solution at the boiling point and distilling hydrogen iodide from the solution.

7 Claims, No Drawings VOLATILIZATION OF IODINE FROM NITRIC ACID USING PEROXIDE BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission. It relates generally to the art of nuclear reactor fuel reprocessing.

As is disclosed in commonly assigned copending applications Ser. Nos. 231,820 and 317,455, a problem encountered in the field of nuclear fuel reprocessing is the removal of radioactive iodine from nitric acid solution. The problem arises from a process wherein spent fuel is dissolved in nitric acid and the desired elements, uranium and plutonium, are separated by solvent extraction from the nitric acid. Radioactive iodine (i.e., I and 1"") is a by-product of the fissioning process and is thus one of the elements present in the nitric acid solution.

From the standpoint of human safety iodine is one of the more dangerous radioactive elements. Iodine is volatile and if ingested tends to concentrate in the thyroid. It is thus desirable to separate and isolate the radioactive iodine from the nitric acid solution prior to further processing in order to protect plant personnel and the environment from possible exposure.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide a process for removing radioactive iodine from nitric acid solution.

It is a further object of this invention to provide a process in which essentially all of the radioactive iodine is removed from the nitric acid solution.

These as well as other objects are accomplished by a process wherein hydrogen peroxide is added to a nitric acid solution containing radioactive iodine. The solution is boiled concurrently with peroxide addition to distill off the iodine as hydrogen iodide.

DETAILED DESCRIPTION According to this invention it has been found that residual traces of radioactive iodine may be removed from nitric acid solution by adding hydrogen peroxide to the solution while concurrently holding the solution at the boiling point to give off hydrogen iodide vapors and collecting the iodine thus given off.

In general, iodine may be removed from nitric acid solution by conventional distillation. However, when the iodine level is reduced to a residual level of about 10 to M or less, the remaining iodine cannot be removed by such conventional distillation techniques. It is at this point where the process of this invention as well as the processes of our copending applications begins. Experimental evidence has indicated that the small amount of iodine which remains in the nitric acid solution is in an oxidized state.

It has been unexpectedly found that hydrogen peroxide appears to act as a reducing agent when added to a nitric acid solution containing iodine. This enables the formation of hydrogen iodide which can be distilled from the solution by boiling. The particular reactions which produce hydrogen iodide are thermodynamically unfavorable; however, it has been found that by keeping the solution at boiling and allowing hydrogen iodide vapors to escape, the reaction will proceed. it is critical to the process of this invention that the solution be kept 2 at boiling or otherwise the reaction which produces hydrogen iodide will not proceed. It is further critical that the hydrogen peroxide be added only at boiling or otherwise the hydrogen peroxide will be lost before the reaction can begin.

The formation of elemental iodine is visually apparent in the process of this invention. Hydrogen iodide is given off as a colorless vapor. As the vapor nears the collection point, it contacts air and immediately oxidizes to release elemental iodine vapor which is distinctively reddish-violet in color.

Many types of distillation arrangements may be used in carrying out the process of this invention. The batch laboratory method used in the examples comprised a 500 ml distillation flask, containing 250 ml solution, fitted with a goose-neck vapor take-off line and means for adding either H 0 or K1 to the Oh is, of course, understood that this would be considerably increased in scale on a production basis.

The process of this invention is generally carried out with a four to six molar nitric acid solution having an iodine molarity of less than 5 X 10 M. Hydrogen peroxide is generally added to the solution as a 6 wt. water solution (about 2 M). However, any concentration of hydrogen peroxide may be used so as to maintain superficially an H 0 concentration after dilution and before reaction or decomposition of at least 4 X 10 M (0.012 wt. percent). Preferably the hydrogen peroxide is added continuously during the process to replace that which is lost due to reaction. The solution is maintained at boiling during the entire process. Boiling generally occurs in the system at a temperature between about to C.

The reaction which occurs to create the hydrogen iodide is thought to be 1 H 0 2H1 T +O A similar reaction is thought to occur with iodine in the higher oxidation states. As pointed out before, this reaction is thermodynamically unfavorable (i.e., AG 0), and the solution must be kept at boiling in order for it to proceed.

The process of this invention is preferably carried out in a stepwise fashion of successive distillations followed by replenishing the solution to its initial volume and initial iodine concentration. Non-radioactive iodine is added to replace the lost radioactive iodine and to act as a carrier for the remaining radioactive iodine. Prior to distillation and hydrogen peroxide addition the solution may be pretreated by bubbling with ozone to minimize the effect of organic impurities which have the effect of tying up the radioactive iodine as organic iodide species. The hydrogen peroxide solution may also be treated with ozone. The hydrogen peroxide also appears to promote isotopic exchange between the nonradioactive iodine diluent and the radioactive iodine which is tied up by the organic species. By this exchange the radioactive iodine may be distilled leaving the non-radioactive iodine with the organic specie. 10-13 Having generally described the invention the following specific examples are given as a further illustration thereof.

EXAMPLE 1 Test solutions of 250 ml of4 M HNO were prepared by adding '1 to provide a radioactive iodine molarity of about 10 M. This solution was boiled at 108C. at a distillation rate of about 0.5 ml/min. Five distillate cuts, each of about 10, were taken and the starting solution was evaluated for iodine content after each such cut. Hydrogen peroxide (30% solution) was added at a rate of 0.25 ml every 15 minutes during the distillation. Makeup water was added to the feed solution after 4 What is claimed is: 1. In a process wherein neutron irradiated substances are dissolved in nitric acid to form a solution and said solution includes radioactive iodine as a solute therein,

each distillate cut was taken. In the case of Run 2, 020- and wherein said iodine is removed from said solution, nated water was utilized for makeup. The results of the improvement comprising removing residual iodine these runs are shown in Table l. The final iodine confrom said solution by: centration was less than 5 X M. boiling said solution;

adding hydrogen peroxide to said boiling solution to TABLE I 10 cause the formation of volat|le hydrogen iodide Resldua' 9 9 and the vaporization of said hydrogen iodide; and

Successive 10% Distillation Cuts Run collecting iodine from said hydrogen iodide. 2nd 3rd 4th 5th 2. The method according to claim 1 wherein said ni- 47 23 [2 tric acid is a four to six molar aqueous solution.

5 3- -3 l5 3. The method according to claim 1 wherein said iodine is present in said solution at a concentration of 5 Normal H,0 makeup X 1() M or [egg hemmed make? 4. The method according to claim 1 wherein said hydrogen iodide is oxidized to form elemental iodine and said l mental iodine is collected. EXAMPLE n e e 5. The method according to claim 1 wherein said step The effect of hydrogen peroxide on the volatilization of boiling is carried out at a temperature within the of iodine from 4 M HMO was studied using solutions r ng of 105C, to 115C. having -5 X 10 M iOdifle Concentration 8 W85 6. The method according to claim 1 wherein said hy- 2.5 X 10" M). In each run the initial solution was 250 drogen peroxide is added to maintain an instantaneous ml and diStillat On Was performed at about hydrogen peroxide concentration of at least 0.012 wt. and the H 0 addition (at the specified concentration) t, was added continuously at about 0.5 ml/min. After 7, Th ethod according to claim 1 comprising the ea h 5 percent d ill i n h ni i l i n nfurther step of isotopically diluting said radioactive iocentration was re-established by adding Kl. The results dine with non-radioactive iodine. are shown in Table II.

TABLE II Residual Iodine (7: Original) After Successive 5% Distillation Cuts H 0 Run Conc. No. (71) Comments 1 st 2nd 3rd 4th 5th 6th 4 3 1.1 0.25 0.14 0.10 0.076 0.066 5 0 control 46 42 37 35 28 6 6 0.31 0.27 0.15 0.l5 0.076 0.060

7 6 b 0.8l 0.l 1 0.059 8 6 a.b 1.3 0.l4 0.037

"Solution 2.5 X 10 M Iodine "Held solution with H1O, being added at 90 for 2 hr. after carrier rcaddition 

1. IN A PROCESS WHEREIN NEUTRON IRRADICTED SUBSTANCES ARE DISSOLVED IN NITRIC ACID TO FORM A SOLUTION AND SAID SOLUTION INCLUDES RADIOACTIVE IODINE AS A SOLUTE THEREIN, AND WHEREIN SAID IODIN IS REMOVED FROM SAID SOLUTION, THE IMPROVEMENT COMPRISING REMOVING RESIDUAL IODINE FROM SAID SOLUTION BY: BOILING SAID SOLUTION, ADDING HYDROGEN PEROXIDE TO SAID BOILING SOLUTION TO CAUSE THE FORMATION OF VOLATILE HYDROGEN IODIDE AND THE VAPORIZATION OF SAID HYDROGEN IODIDE, AND COLLECTING IODINE FROM SAID HYDROGEN IODIDE.
 2. The method according to claim 1 wherein said nitric acid is a four to six molar aqueous solution.
 3. The method according to claim 1 wherein said iodine is present in said solution at a concentration of 5 X 10 4 M or less.
 4. The method according to claim 1 wherein said hydrogen iodide is oxidized to form elemental iodine and said elemental iodine is collected.
 5. The method according to claim 1 wherein said step of boiling is carried out at a temperature within the range of 105*C. to 115*C.
 6. The method according to claim 1 wherein said hydrogen peroxide is added to maintain an instantaneous hydrogen peroxide concentration of at least 0.012 wt. percent.
 7. The method according to claim 1 comprising the further step of isotopically diluting said radioactive iodine with non-radioactive iodine. 